It does depend a bit on what kinds of "key terms" or "important words" you have in mind, but I have had good luck with Google's NLP APIs. They free for small numbers of queries (if memory serves in the thousands per day, but don't quote me on it). It does a good job of identifying people, places, and events, and usually links them to their wikipedia page as well.
You can try it out for free here: https://cloud.google.com/natural-language/ (down at the bottom of the page is a set of test text).
We use it for some experimental features in our social reading app, Bookship. Happy to chat separately about our experience so far...